Fourier transform infrared spectroscopy of the solution-mediated conversion of amorphous calcium phosphate to hydroxyapatite: New correlations between X-ray diffraction and infrared data
Fourier Transform infrared spectroscopic analysis of maturing, poorly crystalline hydroxyapatite (HA) formed from the conversion of amorphous calcium phosphate (ACP) at constant pH or variable pH show only subtle changes in the ν1, ν3 phosphate absorption region (900 cm−1−1200 cm−1). This region is of interest because it can ve detected by analysis of mineralized tissue sections using FT-IR microscopy. To evaluate the subtle spectral changes occurring during the maturation, second derivatives of the spectra were calculated. HA formed at constant pH showed little or no variation in the second derivative peak positions with bands occurring at 960 cm−1, 985 cm−1, 1030 cm−1, 1055 cm−1, 1075 cm−1, 1096 cm−1, 1116 cm−1, and 1145 cm−1. These bands can be assigned to molecular vibrations of the phosphate (PO43−) moiety in an apatitic/stoichiometric environment of HA. In contrast, during the early stages of maturation of the HA formed at variable pH, second derivative peak positions occurring at 958 cm−1, 985 cm−1, 1020 cm−1, 1038 cm−1, 1112 cm−1, and 1127 cm−1 shifted in position with maturation, indicating, that the environment of the phosphate species is changing as the crystals mature. Peaks at 1020 cm−1, 1038 cm−1, 1112 cm−1, and 1127 cm−1 were attributable to nonstoichiometry and/or the presence of acid phosphate-containing species. This concept was supported by the lower Ca:P molar ratios measured by chemical analysis of the synthetic material made at variable pH. Using the second derivative peak positions as initial input parameters, the ν1, ν3 phosphate region of the synthetic HAs prepared at constant pH were curve fit. X-ray diffraction patterns of these same materials were also curve fit to calculate the changes in crystallinty (size/perfection) in the c-axis 002 reflection as well as the 102, 210, 211, 112, 300, 200, and 301 planes. Linear regression analysis showed that the changes in the percent area of the underlying bands at 982 cm−1, 999 cm−1, 1030 cm−1, 1075 cm−1, 1096 cm−1, 1116 cm−1, and 1145 cm−1 were correlated with changes in crystallinity in one or more of the reflection planes. It is suggested that a combination of second-derivative and curve-fitting analysis of the ν1, ν3 phosphate contour allows the most reproducible evaluation of these spectra.
Key wordsFT-IR spectroscopy Hydroxyapatite Second-derivative spectroscopy X-ray diffraction Phosphate species
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- 1.Rey C, Shimizu M, Collins B, Glimcher MJ (1991) Resolution-enhanced Fourier transform infrared spectroscopy study of the environment of phosphate ion in the early deposits of a solid phase calcium phosphate in bone and enamel and their evolution with age: investigations in the ν1PO4 domain. Calcif Tissue Int 49:383–388PubMedGoogle Scholar
- 9.Leung Y, Walters MA, LeGeros RZ (1990) Second derivative spectra of hydroxyapatite. Spectrochim Acta 46A:1453–1459Google Scholar
- 14.Bailey RT, Holt C (1989) Fourier transform infrared spectroscopy and characterisation of biological calcium phosphates. In: Hukins DW (ed) Calcified Tissue, Macmillan Press, London, pp 93–119Google Scholar
- 18.Maddams WF, Mead WL (1982) The measurement of derivative IR spectra-I. Background studies. Spectrochim Acta 38A:437–444Google Scholar
- 19.Hawkes S, Maddams WF, Mead WL, Southon MJ (1982) The measurement of derivative IR spectra-II. Experimental measurements. Spectrochim Acta 38A:445–457Google Scholar
- 20.Maddams WF, Southon MJ (1982) The effect of band width and band shape on resolution enhancement by derivative spectroscopy. Spectrochim Acta 38A:459–466Google Scholar
- 24.Powder Diffraction File (1960) Joint Committee on Powder Diffraction Standards, Philadelphia, PAGoogle Scholar
- 26.Berry EE, Baddiel CB (1967) Some assignments in the infra-red spectrum of octacalcium phosphate. Spectrochim Acta 23A: 1781–1792Google Scholar
- 27.Berry EE, Baddiel CB (1967) The infra-red spectrum of dicalcium phosphate dihydrate (Brushite). Spectrochim Acta 23A: 2089–2097.Google Scholar
- 28.van Wazer JR (1958) Phosphorus and its compounds. Interscience, New York.Google Scholar
- 29.Cullity BD (1956) Elements of x-ray diffraction. Addison-Wesley Publishing Company Inc., Reading, Ma. pp. 97–99Google Scholar